The northern plains of Texas, where I lived as a boy before my parents moved (and fortunately took me with them) to Canada, are in “Tornado Alley,” the area of the United States where most of its 770 yearly tornadoes occur. So is Harding University in Searcy, Arkansas, where I went to college. There, in addition to the traditional guides to fire exits, all buildings included posters telling you where to go in the event of a tornado.
When we came to Saskatchewan in 1967, we were told Saskatchewan doesn’t have tornadoes–only the occasional “twister.”
Well, a rose by any other name would smell as sweet, and nowadays nobody tells us that Saskatchewan doesn’t have tornadoes. Fact is, it does. Ask Peebles. (Ask Regina, for that matter–they may have called it the Regina Cyclone, but it was a twist–um, that is, a tornado.)
But what is a tornado, and how does it form?
According to my science dictionary, a tornado is “the most violent kind of storm; an intense whirlwind of small diameter . . . generally funnel-shaped.” Only the swirling winds around the eye of a hurricane can compare in brute force, but hurricanes are huge, lumbering beasts. Tornadoes can form in moments, tear across the landscape as fast as a car for 15 or 20 minutes, and then vanish, leaving behind destruction and, sometimes, death.
Fortunately, tornadoes are generally rare, although they do occur worldwide, particularly in the U.S. and Australia in the spring and summer. They even occur at sea and over lakes, where they form waterspouts–tornadoes that are sheathed in spray.
Meteorologists don’t fully understand tornadoes, making it difficult to predict exactly where and when they’ll occur. The sequence of events producing a tornado has never been accurately simulated in a laboratory or computer.
What is known is that the most severe thunderstorms occur where air masses of very different temperature and humidity collide, especially when dry, cooler air piles up on top of warm, moist air. Normally the cooler air simply sinks through the warmer air while the warm air rises. This usually produces clouds and rain.
But sometimes the difference in temperature between the air masses is so slight the moist, warmer air won’t rise. If something then breaks the cap of dry air–say, heat from the sun-baked ground finally warms the moist air enough for it to escape–a severe thunderstorm may form in moments. As moisture rushes skyward, it condenses into clouds, releasing energy that further warms the rising air and accelerates its rate of climb.
This sucks more warm air in from underneath, so powerfully the upward-rushing winds may reach 160 kilometres per hour. Giant thunderstorms called supercells may last for several hours, soar 18,000 metres high, drop three centimetres of rain in ten minutes, blanket the ground with huge hail–and spawn tornadoes.
The leading theory concerning tornado formation invokes wind shear, a difference in wind speed or direction at various heights. Usually in big thunderstorms the low-altitude wind is moderate and heads southeast; higher up the wind is stronger and more westerly.
This wind shear sets the air surrounding the storm’s centre rotating and creates narrow horizontal tubes of spiralling air. The surface air spinning into the storm bumps these tubes, tilting them up.
As more air is drawn inward the storm becomes more concentrated and turns faster, just as a slowly turning figure skater spins faster if she pulls in her arms. The hidden core of the storm may spin as fast as 100 kilometres per hour. As it slowly grows downward through the cloud base it creates a “wall cloud” that can be seen turning beneath the storm.
Generally the storm runs out of moist surface air at this point and dies. But if it doesn’t, that rotating cloud will continue stretching toward the ground; and sometimes one of those small, spiralling columns of air created by wind shear starts rotating along the edge of the wall cloud and forms a funnel. Sometimes the entire wall cloud transforms into a truly giant tornado a kilometre or more wide.
“All” a tornado is, then, is a column of spinning air–but air spinning so fast, at about 600 kilometres an hour, that at the centre of the column is an area of extremely low pressure. Water vapor in this area of low pressure expands and cools, becoming visible, which is why tornadoes, especially if they don’t touch down, are white or gray. Once they touch down, of course, they suck up dust and other material and often turn quite black.
People in tornado-prone areas, by the way, have often been taught to open the windows in the event of a tornado, on the theory that the house will explode because of the sudden drop in air pressure outside. Unfortunately, the latest research indicates that opening the windows actually worsens the situation–the high speed winds lower the pressure on the leeward walls, tugging them outward, and open windows allow those same winds to blow through the house and add even more pressure. The result: the house topples. (“The pictures we saw looked like houses exploding,” says a National Weather Service spokesman, somewhat defensively.)
All weather forecasters can do is warn the public when conditions favor tornado formation. To accurately predict tornadoes, they need more data.
To that end, meteorologist Howard Bluestein of the University of Oklahoma has spent part of each of the last 14 years actually chasing tornadoes, striving to get his instruments (and therefore himself) as close as possible. (He’s been fascinated with violent weather ever since, as a small child, he saw the family TV blow up during a lightning storm.)
All through tornado season, Bluestein watches for favorable conditions; then, in a truck loaded with equipment and assistants, he chases thunderstorms, searching for tell-tale tornado-weather signs: supercell, wall cloud, heavy hail. He’s seen lots of tornadoes, but not as many as he’d like. Some years he sees none at all.
Most people would consider that a blessing, but not Bluestein. “Nice and juicy up there!” he’ll shout gleefully as the sky turns black. When a tornado fails to materialize, his spirits sag. “We know a lot more than we did 15 years ago, but there are major gaps in our knowledge,” he says. “We don’t really know why tornadoes occur.”
He’s out to change that by chasing them down.
Try to tell him that science is boring.